An explanation will be given below to a constitution of a fuel vapor drain control system 101 according to the conventional art with reference to FIG. 6. FIG. 6 is a view illustrating a schematic constitution of the fuel vapor drain control system 101 for controlling a fuel vapor (referred below to as vapor containing a mixed gas of air and a vaporized fuel) in a fuel tank according to the prior art, in an automobile provided with an engine, in which gasoline, gas oil or the like is used as fuel (feeding state).
A fuel supply section including a fuel tank 102 and the fuel vapor drain control system 101 in an automobile is an important safety part under the application of safety regulations concerning fuel leakage at the time of collision, pressure control of fuel vapor generated in a fuel tank due to vibrations during running of a vehicle and changes in environmental temperature, or the like and under the application of law regulations such as pollution control for prevention discharging of fuel vapor to the atmosphere, or the like, and must be taken account in various points to prevent leakage of a fuel even in the case where a vehicle is inclined and circles round, from the viewpoint of safety.
102 denotes a fuel tank, and 103 a loading arm having a fill opening 103a and extending into the fuel tank 102. Arranged above the fuel tank 102 are a fuel vapor discharge device 104 and fuel shut-off valves 105a, 105b, which prevent excessive oil feed at the time of feed.
The fuel vapor discharge device 104, of which cross sectional construction is shown in detail in FIG. 7, is composed of a differential pressure regulating valve 104a on an upper side thereof and a float valve 104b on a lower side thereof, and both ends of a communication hole 104c disposed on an upper portion of the fuel tank 102 serve as seat surfaces, opening and closing states of the communication hole 104c being enabled to be controlled by a diaphragm 104d and a float 104e. 
In addition, in order to illustrate the operation of the differential pressure regulating valve 104a and the float valve 104b on the lower side of the device, a state, in which the differential pressure regulating valve 104a is opened and the float valve 104b is closed, is shown on a left side of an axis A1—A1 of the fuel vapor discharge device 104 in FIG. 7, and a state, in which the differential pressure regulating valve 104a is closed and the float valve 104b is opened, is shown on a right side of the axis.
In order to actuate the diaphragm 104d of the differential pressure regulating valve 104a, there are provided chambers 104f, 104g partitioned by the diaphragm 104d. Connected to the chamber 104f is a signal line 106 extended from a midway of the loading arm 103, and introduced into the chamber 104g through the communication hole 104c is pressure in the fuel tank 102 (when the diaphragm 104d is opened), a vent line 108 being connected to a canister 107.
With the float valve 104b of the fuel vapor discharge device 104, when a liquid level rises and the device inclines/tumbles, the float 104e floats to close the communication hole 104c, thus enabling preventing a fuel from leaking from the fuel tank 102.
Connected to the fuel shut-off valves 105a, 105b is an evaporative line 109 (vaporized fuel passage) for preventing an increase in internal pressure in the fuel tank 102, the evaporative line 109 being communicated to the canister 107. The evaporative line 109 is closed by floats 105c, 105d at the time of turning/fluctuation in running.
The provision of the two fuel shut-off valves 105a, 105b enables internal pressure in the fuel tank 102 to escape through one of the fuel shut-off valves even if the other of the fuel shut-off valves is closed when a vehicle inclines.
A check valve 110 is provided on the evaporative line 109 to prevent a fuel from flowing directly into the canister 107 or a reservoir chamber possibly provided between the check valve 110 on the evaporative line 109 and the canister 107, the reservoir chamber being not shown. Pressure, at which the check valve 110 is opened, is set low in order to prevent an increase in internal pressure in the fuel tank 102.
Also, a test line 111 is provided on the evaporative line 109 to bypass the check valve 110, and a solenoid valve 112 is provided midway the test line 111 to open and close the test line 111.
The canister 107 is connected to a scavenging means 113 to be able to feed the absorbed fuel vapor to a side of an engine E for burning.
With the conventional fuel vapor drain control system 101 constructed in the above manner, the differential pressure regulating valve 104a is opened by a difference between pressure in the tank and outside air pressure (the signal line 106) at the time of feed, so that a gas (mainly, air and fuel vapor) in the fuel tank 102 is absorbed by the canister 107 via the vent line 108.
When the tank becomes full, the float valve 104b is closed to shut-off the vent line 108, thus temporarily increasing internal pressure in the tank to raise a liquid level (a liquid level indicated by H in FIG. 6) in the loading arm 103 to actuate the automatic stopping function of an oil filler gun G.
With the above fuel vapor drain control system 101, however, the float valve 104b shifts to a closed state from an opened state at a stroke in that closing action of the float valve 104b of the fuel vapor discharge device 104, which is caused by the ascent of a liquid level at the time of feed, so that internal pressure in the tank rapidly increases to excessively raise a liquid level (a liquid level indicated by H1 in FIG. 6) in the loading arm 103.
In this state, since even when the float valve 104b is put in a closed state, the fuel shut-off valves 105a, 105b cause internal pressure in the fuel tank 102 to gradually escape, the liquid level lowers after the lapse of some time to dissolve the above excessive ascent but the liquid level (H1) temporarily approaches the fill opening 103a. 
With a view to solving such problem, the fuel vapor discharge device 104 in some cases comprises a configuration, as shown in FIG. 8, which throttles the flow rate of a gas discharged prior to closing of the float valve 104b to gradually raise internal pressure in the tank. In addition, in order to illustrate the operation of a flowrate throttle member 104h and the float valve 104b, a state, in which the float valve 104b is opened and the flowrate throttle member 104h throttles the flow rate, is shown on a left side of an axis A2—A2 of the fuel vapor discharge device 104 in FIG. 8, and a state, in which the float valve 104b is opened and the flowrate throttle member 104h does not throttle the flow rate, is shown on a right side of the axis.
In FIG. 8, the flowrate throttle member 104h is provided above and connected to the float 104e by a spring 104i. Normally, a fuel vapor is discharged from two discharge holes 104j, 104k, and when the float valve 104b rises as a liquid level ascends at the time of feed, the flowrate throttle member 104h closes one 104j of the discharge holes prior to closing of the float valve 104b to throttle the flow rate of fuel vapor as discharged, and finally the float 104e closes the other 104k of the discharge holes.
With the configuration shown in FIG. 8, however, the spring 104i is provided between the flowrate throttle member 104h and the float 104e, whereby a position in the fuel vapor discharge device 104, in which the float 104e is mounted, is lowered, and so a liquid level is lowered when the fuel tank 102 becomes full at the time of closing of the float valve 104b, thereby causing a problem that it is not possible to enlarge a full-tank capacity.
Also, with a conventional fuel vapor drain control system 101′ shown in FIG. 9, an evaporative line 109 provided with a two-way valve for preventing an increase in internal pressure in a fuel tank 102 is connected to a canister 107 from one fuel shut-off valve 105. With the fuel vapor drain control system 101′ shown in FIG. 9, when fuel enters into the evaporative line 109 from the fuel shut-off valve 105 at the time of turning/fluctuation in running, it is feared that the fuel having once entered remains in the evaporative line 109 to be shut-off from escape and finally passes through the two-way valve to flow to the canister 107.
The present invention solves the above problems of the conventional art, and has its object to suppress or prevent a liquid level in a loading arm to excessively ascend (in some cases, a fuel possibly spills) in a full-tank state with feed.